This invention relates to methods and apparatus for synchronizing electrical signals together; and more particularly to methods and apparatus for synchronizing an input signal that consists of a plurality of bits which nominally are of one predetermined period but which actually are distorted therefrom, with the local generator signal that consists of a plurality of undistorted bits of the same predetermined period. Such methods and apparatus are useful for example, in communication systems which transmit information to remote stations through the air by radio signals. At the stations, it is necessary for several reasons to synchronize a locally generated signal with the received information signals. For example, externally generated noise operates to distort the bits from their nominal period. That is, the bits which are received typically are either stretched or shrinked from their nominal period. Thus, by synchronizing a locally generated signal that consists of a plurality of non-distorted bits having the same predetermined period, the center of the bits which are received can be determined. Then the information contained therein can be sampled at that point.
Ideally, the phase of the locally generated signal should be generated such that the sum of all phase differences between corresponding bits in it and input signals equals zero. One way to accomplish this is to generate a local generator signal with a phase that is arbitrary with respect to the input signal; to measure the phase differences between it and the input signal for a relative large number of bits; and then to shift this signal by an amount equal to the average of all the measured phase differences. To do this however, requires a relatively large amount of memory. That is, each of the measured phase differences must be stored in order that they can subsequently be averaged; and to get a good average, requires that the number of phase measurements made must be relatively large.
Another problem with the above averaging approach is that the synchronization time is relatively long. This because no phase correction occurs until after a relatively large number of phase measurements have been made, and the average phase difference has been calculated.
Therefore, in the prior art synchronization of electrical signals is performed by other methods and apparatus which are simpler to implement, but which do not produce ideally synchronized signals. For example, typical prior art methods and apparatus operate in a manner wherein the phase difference between some relatively small number (such as 3 or 4) of the most recent bits in the input signal and the local generator signal are used to determine the amount of phase correction to be made. Generally, these signal synchronizers include a capacitor for dynamically storing the last several phase measurements made. This capacitor is slightly charged or discharged with each successive phase measurement.
Thus, the capacitor voltage is representative of the phase differences between a relatively small number of bits in the local generator signal and input signal. The capacitor voltage is then utilized to control the amount by which the local generator signal should be shifted in phase to generate a new local generator signal which is more closely synchronized to the input signal.
These prior art devices however, have so called "sensitivity" problems. That is, if the storage time of the capacitor is relatively small then the locally generated signal follows too closely the distortion within the most recently received information bits. Conversely, if the storage time of the capacitor is relatively large, then the locally generated signal is unable to follow or correct for phase changes between it and the input signal. In either case, these devices do not track the input signal based on the average of all prior phase measurements made.
Therefore, it is one object of the invention to provide an improved method of the synchronizing a locally generated signal with a distorted input signal.
Another object of the invention is to provide improved apparatus for synchronizing a locally generated signal with a distorted input signal.
Still another object of the invention is to provide a method and apparatus for generating a signal such that the sum of all phase differences, between it and a distorted input signal are zero.